Most Common Analytical Techniques for the Characterization of Nanomaterials

TRANSMISSION ELECTRON MICROSCOPY (TEM)

In this technique, a beam of electrons is transmitted through a sample to form an image. There is access to the internal structure of the matter.

Excellent for visualizing samples and producing representative images, evaluation of particle shape, size and size distribution. Also enables visualization of the degree of aggregation / agglomeration of a sample, and possibility to investigate the crystalline phase and perform chemical analysis if an Energy-dispersive X-ray spectroscopy (EDX) device is coupled with the microscopy instrument.

SCANNING ELECTRON MICROSCOPY (SEM)

This technique delivers images with information about the topography and composition of sample, by scanning the surface with a focused beam of electrons.

Depending on the microscope and also of the size of the nanomaterials, this technique can also be used to determine size, size distribution, shape and degree of aggregation/agglomeration of the particles. EDX can also be coupled with this microscope. Excellent technique to observe nanomaterials deposited on surfaces of different materials (e.g. polymers, textile fibers), and to assess their spatial distribution in the matrix.

DYNAMIC LIGHT SCATTERING (DLS)

DLS measures the Brownian motion of (nano)particles and relates it with their hydrodynamic size and is applied to measure the size of stable dispersion.

DLS is a routine technique that measures the hydrodynamic particle size distribution of liquid dispersions of nanomaterials (typically in the range of 5 to 1000 nm). The diameter calculated from the diffusional properties of the particle will be indicative of the apparent size of the dynamic hydrated/solvated particle, which is as a rule bigger than the real diameter shown by the particles (i.e. that determined by electron microscopy). This technique is also useful to assess stability over time and to evaluate the influence of the addition of stabilizers and other additives in the stability of the nanomaterial dispersions.

ZETA POTENTIAL

The Z-potential is calculated through the electrophoretic mobility, which is related to particle stability and the behaviour of a particle in solution, and is measured by DLS. This measurement gives information about the stability of particle dispersions, due to the fact that it is related to surface charge.

NANOPARTICLE TRACKING ANALYSIS (NTA)

This technique detects the Brownian motion of the particles (as DLS) and is able to measure individual particles by image analysis of the bright spots generated from light scattering. For polydisperse samples, Nanosight performs better than DLS, since DLS produces an average particle size due to the ensemble measurement (all particles measured at the same time) and is biased towards larger particles within the sample (larger particles scatter light more intensely than smaller ones).

GAS SORPTION FOR SURFACE AREA ANALYSIS (BET)

BET is based on the physical multi-layer adsorption of usually non-corrosive gases (e.g. nitrogen, argon) on solid materials (e.g powders) to determine the specific surface. The data collected is displayed in the form of a BET isotherm (Figure 5), which plots the amount of gas adsorbed as a function of the relative pressure (p/po).

Gas sorption analysis is used to determine the surface area (m²/g) and the porosity of powders. The size of the particles can also be estimated from the surface area, if the density of the material is known.

INDUCTIVELY COUPLED PLASMA MASS SPECTROMETRY (ICP-MS)

The main advantages of this technique lie in the high accuracy, low detection limits (ng/L) and low economic cost, analyzing most of the elements and isotopes present in the periodic table simultaneously in a very short time.

ICP-MS is an essential analytical technique to determine the inorganic content of a sample containing nanomaterials (e.g. Ag, CuO, ZnO, CeO2, TiO2). It could also be used to characterize the purity of a nanomaterial.

X-RAY DIFFRACTION (XRD)

XRD is a non-destructive and non-contact technique where a diffraction pattern can be obtained for a sample when X-rays interact with crystalline materials, typically loose powders, thin films, polycrystalline and bulk materials. Particularly, X-ray powder diffraction can be used to characterize various types of micro- and nano-crystalline materials, including inorganics, organics, drugs, minerals, zeolites, catalysts, metals and ceramics. The amount of information to be obtained depends on the nature of the sample microstructure (crystallinity, structure imperfections, crystallite size, texture), the complexity of the crystal structure (number of atoms in the asymmetric unit cell, unit cell volume) and the quality of the experimental data (instrument performances, counting statistics).

The main use of powder diffraction is to identify components in a sample by a search/match procedure. XRD data can be used to obtain phase composition, crystallite size, lattice strain, and crystallographic orientation, depending on the sample.

FOURIER-TRANSFORM INFRARED SPECTROSCOPY (FTIR)

FTIR spectroscopy can be used to determine the presence of characteristic functional groups of coating or impurities on the sample. The spectrum of a material serves as a fingerprint which can be useful in identifying unknown samples.

UV-VISIBLE SPECTROSCOPY (UV-vis)

The main output of this technique is the absorbance spectrum; absorbance of UV-Vis radiation with respect to the wavelength. The UV-vis absorbance of nanomaterials is like a finger print, and can be used to detect what material the particles are made of.

UV-vis is a tool that can detect the optical properties of nanomaterials that are related to size, shape, concentration, agglomeration state, and refractive index near the nanoparticle surface.

THERMOGRAVIMETRIC ANALYSIS (TGA)

The main principle of TGA is that mass change of a sample can be studied under programmed conditions, and can be used to assess absorption, adsorption, desorption, vaporization, sublimation, decomposition, oxidation, and reduction. It can also be used to evaluate the percentage of organic/inorganic matter of the nanomaterial samples.